Within evolutionary biology, signalling theory refers to the scientific theory around how organisms signal their condition to others. The central concept is that of honest signalling — that organisms should honestly signal their good condition to others rather than spending their energy elsewhere i.e. it is the evolutionarily stable strategy (ESS) and the population is not susceptible to invasion by cheats who signal less than their optimum. The handicap principle is one influential idea — proposed by Amotz Zahavi — that honest signals are maintained at an ESS by being wastefully costly. Others, such as John Maynard Smith, believe that signals can be both honest and cost-free.

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Sam Brown and W.D. Hamilton proposed the idea that autumnal leaf colouration was trees signalling to aphids and other pests. Autumn colour is costly to trees, aphids appear to preferentially avoid trees with bright leaves and tree species with bright leaves have more specialist aphid pests than do trees lacking bright leaves.